Cdk1 promotes cytokinesis in fission yeast through activation of the septation initiation network.

In Schizosaccharomyces pombe, late mitotic events are coordinated with cytokinesis by the septation initiation network (SIN), an essential spindle pole body (SPB)-associated kinase cascade, which controls the formation, maintenance, and constriction of the cytokinetic ring. It is not fully understood how SIN initiation is temporally regulated, but it depends on the activation of the GTPase Spg1, which is inhibited during interphase by the essential bipartite GTPase-activating protein Byr4-Cdc16. Cells are particularly sensitive to the modulation of Byr4, which undergoes cell cycle-dependent phosphorylation presumed to regulate its function. Polo-like kinase, which promotes SIN activation, is partially responsible for Byr4 phosphorylation. Here we show that Byr4 is also controlled by cyclin-dependent kinase (Cdk1)-mediated phosphorylation. A Cdk1 nonphosphorylatable Byr4 phosphomutant displays severe cell division defects, including the formation of elongated, multinucleate cells, failure to maintain the cytokinetic ring, and compromised SPB association of the SIN kinase Cdc7. Our analyses show that Cdk1-mediated phosphoregulation of Byr4 facilitates complete removal of Byr4 from metaphase SPBs in concert with Plo1, revealing an unexpected role for Cdk1 in promoting cytokinesis through activation of the SIN pathway.

Identification of new players in cell division, DNA damage response, and morphogenesis through construction of Schizosaccharomyces pombe deletion strains.

Many fundamental biological processes are studied using the fission yeast, Schizosaccharomyces pombe. Here we report the construction of a set of 281 haploid gene deletion strains covering many previously uncharacterized genes. This collection of strains was tested for growth under a variety of different stress conditions. We identified new genes involved in DNA metabolism, completion of the cell cycle, and morphogenesis. This subset of nonessential gene deletions will add to the toolkits available for the study of biological processes in S. pombe.

RACK1/Asc1p, a ribosomal node in cellular signaling.

RACK1/Asc1p and its essential orthologues in higher eukaryotes, such as RACK1 in metazoa, are involved in several distinct cellular signaling processes. The implications of a total deletion have never been assessed in a comprehensive manner. This study reveals the major cellular processes affected in a Saccharomyces cerevisiae Δasc1 deletion background via de novo proteome and transcriptome analysis, as well as subsequent phenotypical characterizations. The deletion of ASC1 reduces iron uptake and causes nitrosative stress, both known indicators for hypoxia, which manifests in a shift of energy metabolism from respiration to fermentation in the Δasc1 strain. Asc1p further impacts cellular metabolism through its regulative role in the MAP kinase signal transduction pathways of invasive/filamentous growth and cell wall integrity. In the Δasc1 mutant strain, aberrations from the expected cellular response, mediated by these pathways, can be observed and are linked to changes in protein abundances of pathway-targeted transcription factors. Evidence of the translational regulation of such transcription factors suggests that ribosomal Asc1p is involved in signal transduction pathways and controls the biosynthesis of the respective final transcriptional regulators.

5'TRU: identification and analysis of translationally regulative 5'untranslated regions in amino acid starved yeast cells.

We describe a method to identify and analyze translationally regulative 5'UTRs (5'TRU) in Saccharomyces cerevisiae. Two-dimensional analyses of (35)S-methionine metabolically labeled cells revealed 13 genes and proteins, whose protein biosynthesis is post-transcriptionally up-regulated on amino acid starvation. The 5'UTRs of the respective mRNAs were further investigated. A plasmid-based reporter-testing system was developed to analyze their capability to influence translation dependent on amino acid availability. Most of the 13 candidate 5'UTRs are able to enhance translation independently of amino acids. Two 5'UTRs generally repressed translation, and the 5'UTRs of ENO1, FBA1, and TPI1 specifically up-regulated translation when cells were starved for amino acids. The TPI1-5'UTR exhibited the strongest effect in the testing system, which is consistent with elevated Tpi1p-levels in amino acid starved cells. Bioinformatical analyses support that an unstructured A-rich 5' leader is beneficial for efficient translation when amino acids are scarce. Accordingly, the TPI1-5'UTR was shown to contain an A-rich tract in proximity to the mRNA-initiation codon, required for its amino acid dependent regulatory function.

The Saccharomyces homolog of mammalian RACK1, Cpc2/Asc1p, is required for FLO11-dependent adhesive growth and dimorphism.

Nutrient starvation results in the interaction of Saccharomyces cerevisiae cells with each other and with surfaces. Adhesive growth requires the expression of the FLO11 gene regulated by the Ras/cAMP/cAMP-dependent protein kinase, the Kss1p/MAPK, and the Gcn4p/general amino acid control pathway, respectively. Proteomics two-dimensional DIGE experiments revealed post-transcriptionally regulated proteins in response to amino acid starvation including the ribosomal protein Cpc2p/Asc1p. This putative translational regulator is highly conserved throughout the eukaryotic kingdom and orthologous to mammalian RACK1. Deletion of CPC2/ASC1 abolished amino acid starvation-induced adhesive growth and impaired basal expression of FLO11 and its activation upon starvation in haploid cells. In addition, the diploid Flo11p-dependent pseudohyphal growth during nitrogen limitation was CPC2/ASC1-dependent. A more detailed analysis revealed that a CPC2/ASC1 deletion caused increased sensitivity to cell wall drugs suggesting that the gene is required for general cell wall integrity. Phosphoproteome and Western hybridization data indicate that Cpc2p/Asc1p affected the phosphorylation of the translational initiation factors eIF2 alpha and eIF4A and the ribosome-associated complex RAC. A crucial role of Cpc2p/Asc1p at the ribosomal interface coordinating signal transduction, translation initiation, and transcription factor formation was corroborated.

Activation of the ADE genes requires the chromatin remodeling complexes SAGA and SWI/SNF.

The activation of the ADE regulon genes requires the pair of transcription factors Bas1 and Pho2. In a genome-wide screen for additional regulators of the pathway, strains with mutations in multiple subunits of the chromatin remodeling complexes SAGA and SWI/SNF were uncovered. These mutants exhibited decreased expression of an ADE5,7-lacZ reporter and native ADE compared to the wild-type strains, but the expression of the BAS1 and PHO2 genes was not substantially decreased. An unregulated Bas1-Pho2 fusion protein depended upon SAGA and SWI/SNF activity to promote transcription of a reporter. A significant but low-level association of Gcn5-myc and Snf2-myc with the ADE5,7 promoter was independent of adenine growth conditions and independent of the presence of the activator proteins Bas1 and Pho2. However, the increase in occupancy of Bas1 and Pho2 at ADE5,7 depended on both SAGA and SWI/SNF. The loss of catalytic activity of both SAGA and SWI/SNF complexes in the gcn5Delta snf2Delta double mutant was severely detrimental to ADE-lacZ reporter expression and native ADE gene expression, indicating complementary roles for these complexes. We conclude that Bas1 and Pho2 do not recruit the SAGA and SWI/SNF complexes to the ADE5,7 promoter but that the remodeling complexes are necessary to increase the binding of Bas1 and Pho2 in response to the adenine regulatory signal. Our data support the model that the SAGA and SWI/SNF complexes engage in global surveillance that is necessary for the specific response by Bas1 and Pho2.